The R-T-B based permanent magnet (R represents a rare earth element, T represents Fe or Fe with part of it replaced with Co, and B represents boron) comprising a tetragonal compound R2T14B as the major phase is known to have excellent magnetic properties and has been considered as a representative permanent magnet with good performances since it was invented in 1982 (Patent Document 1: JP Sho59-46008).
In particular, the R-T-B based magnets in which the rare earth element R consists of Nd, Pr, Dy, Ho or Tb have large magnetic anisotropy fields Ha and are preferred as materials for permanent magnets. Especially the Nd—Fe—B based magnet having Nd as the rare earth element R is widely used in people's livelihood, industries, conveyer equipment and the like, because it has a good balance among saturation magnetization Is, curie temperature Tc and magnetic anisotropy field Ha and is better in resource volume and corrosion resistance issues than R-T-B based permanent magnets with other rare earth elements.
The motors used in people's livelihood, industries, conveyer equipment and the like, most of which are required to have variable speeds, are mainly inverter driven by induction motors with simple constructions and low prices. However, recently, in order to save energy, people tend to use the permanent-magnet synchronous motor which has a higher efficiency than the induction motor.
In the permanent-magnet synchronous motor, a permanent magnet is disposed in the rotor and the used permanent magnet is one having one third of the masses occupied by the rare earth element Nd with a big specific gravity (just as the Nd—Fe—B based magnet), causing increased inertia momentum due to the elevated weight of the rotor. That is, the controllability and efficiency are deteriorated.